The present invention relates to orthopedic implantation, particularly implantation of prosthetic devices to repair or replace hard tissue of warm-blooded mammals, e.g., bones and joints of humans and animals which processes utilize a bone cement for fixation of the prosthesis.
Substantial effort has been previously expended in the area of orthopedic implantation of prostheses in both human and animals, attempting to repair or replace a diseased or damaged bone and/or joint associated with the bone. In general, orthopedic implantation of prostheses has been fraught with various problems. Specifically, problems have revolved around adequacy of strength characteristics of the prosthesis as well as the physiological impact and effect of the implant on the patient under the influence of normal body functions, tissue, bone, body fluids and the like.
Ideally, the prosthetic implant should be designed to closely assimilate, as much as possible, the characteristics of the bone and/or joint that the implant is intended to repair and/or replace. In particular, the implant should possess adequate structural strength to undergo compressive stress, flexural stress, and the like imparted thereto during normal body function without reaching the point of failure. In like vein, the implant must be resistant to corrosion and other degradation in vivo and should be virtually inert to body fluids. Further the means for securing the implant within the bone should not adversely affect the surrounding body tissue or create any adverse physiological impact on the body system of the patient.
In order to achieve optimization of implant design, prior techniques have involved the manufacture of implants, or prostheses from various materials, each of which has been engineered to possess one or more of the above requisite characteristics for a prosthesis. Generally, prosthetic implants have been manufactured of various metal alloys, ceramics, polymeric materials, mixtures of polymeric materials with metals and/or ceramics, and the like. Certain prostheses have been manufactured with particular porous coatings, normally polymeric or ceramic in nature, attempting to improve fixation by permitting bone tissue growth within porosities of the porous surface.
One widely accepted implantation technique, particularly for joint replacement, especially total hip arthroplasty, utilizes bone cement, generally an acrylic, that is premixed and placed into a prepared bone cavity in which the implant is to be fixed followed by location of the prosthesis in the bone cement and ambient curing or polymerization of the cement. Though the use of bone cement for orthopedic implantation is a generally accepted procedure, certain problems exist concerning the use of same in the implantation technique, the physiological impact on the surrounding body area, and expected useful life of the implant. Bone cement conventionally includes an acrylic polymeric powder which is pre-mixed with a liquid acrylic monomer system to provide a doughy mass. The doughy mass is inserted into the bone cavity.
Polymerization of the bone cement is exothermic in nature, generating temperatures in vivo in excess of 60.degree. C. which can result in necrosis of surrounding body cells. The monomer system is basically toxic in nature and can interfere with the systemic function of the patient, leading to a decrease in blood pressure, and perhaps in certain circumstances, more serious cardiovascular problems. Normal use of the acrylic bone cement can also lead to further problems. After the doughy cementitious mixture has been placed within the prepared bone cavity, the surgeon only has from about two to five minutes during which the prosthesis may be properly located in the bone cavity surrounded by the bone cement. There is thus very little available time for the surgeon to ensure precise placement of the prosthesis. Additionally, the bone cement shrinks during polymerization, possibly leading to the production of both microscopic and macroscopic gaps along the interface between the prosthesis and the cement and/or the cement and the bone tissue, leading to a possibly inferior implantation. In the event of improper placement of the prosthesis, same cannot be merely replaced after polymerization of the bone cement. Instead it is necessary to forcefully remove the prosthesis, drill out the polymerized bone cement, and re-prepare the bone cavity. Not only is the repeat implantation inconvenient to the patient, but there is always the danger of trauma prompted by the additional reaming of the bone cavity. Further, extensive intermedullary cavity preparation can block bone sinusoids, enhancing the probability of tissue necrosis and fat embolism.
The present invention overcomes or substantially reduces the incidence of the problems set forth above with respect to the use of bone cement for surgical implantation of a prosthesis. While many different techniques are set forth in the prior art concerning implantation by the use of bone cement, as exemplified by the following listed patents, none of the prior art is believed to anticipate or suggest the subject matter of the present invention.
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